The SMC X-ray binary SXP4.78: A new Type II outburst and the identification and study of the optical counterpart

SXP4.78 was originally discovered in 2000 as a pulsar in the Small Magellanic Cloud by the Rossi X-ray Timing Explorer, but it was not spatially located at that time. A new detection in 2018 with the Neil Gehrels Swift Observatory during a Type II outburst permitted its position to be accurately located and its optical counterpart to be identified.We report X-ray and optical monitoring covering epochs before and during the outburst. Using photometric data, we show the long-term variability of the Be disc where we present flux and colour changes associated with the disc growth and decay over a period of ∼ 6000 d. We show evidence of disc growth during the recent outburst through an increase in the HÁ equivalent width and photometric flux. Period analysis was performed using both optical photometric and spectroscopic data, but with no significant detection of an orbital period. A modest periodic signature of 2.65 d was detected from the Optical Gravitational Lensing Experiment (OGLE) I-band data, but we attribute that to the non-radial pulsations of the Be star.We also obtained a blue spectrum from the Southern African Large Telescope, which permits us to classify the spectral type as B0.5 IVCV.